This invention relates to particulate matter monitors, and more particularly to monitors for continuous monitoring of particulate matter.
Ambient particles in the size range 2.5 to 10 xcexcm are referred to as coarse particles or coarse mode (CM) aerosols. Coarse particles may include several potentially toxic components, such as resuspended particulate matter from paved and unpaved roads, industrial materials, brake linings, tire residues, trace metals, and bio-aerosols such as anthrax. Since a considerable fraction of these particles may deposit in the upper airways and to a lesser extent into the lower airways, they may exacerbate health conditions such as asthma or possibly cause health problems as in the case of bio-aerosols. Recent data from a small number of epidemiological studies indicate that, apart from or in addition to the fine fraction (FM) of particulate matter (also called PM2.5), health effects also may be closely associated with the CM fraction and sometimes even to a larger extent than FM. In vitro studies with human monocytes suggest that cellular toxicity and inflammation also may be associated with the CM and its biological components.
Current measurements of both the PM10(particulate matter having an aerodynamic diameter that is less than 10 um) and PM2.5(particulate matter having an aerodynamic diameter that is less than 2.5 um) mass concentrations are generally based on gravimetric analysis of particles collected on filters over a period of 24 hours. Gravimetric analysis is generally used because most of the particle data used for the epidemiological studies investigating associations between mortality and morbidity outcomes and ambient particle exposures are based on PM concentrations. Typically, a time-integrated sample (e.g., over 24 hours) is collected on the filter, which is later equilibrated at designated temperature and RH conditions, and subsequently weighed to determine the mass of the deposited PM. Dividing by the amount of air sample yields the atmospheric concentration. Since the values of atmospheric parameters influencing ambient particle concentration, hence human exposure, such as the emission strengths of particle sources, temperature, RH, wind direction and speed and, mixing height, fluctuate in time scales that are substantially shorter than 24 hours, a 24-hour measurement may not reflect an accurate representation of human exposure. Thus, more accurate, better quality data on the physical-chemical characteristics of particles are needed to understand their atmospheric properties and health effects.
Techniques that are capable of providing continuous or near continuous measurements (i.e. 1-hour average or less) are highly desirable because they can provide accurate information on human exposure and atmospheric processes in short timer intervals. Over the past decade, several methods have been developed for continuous PM10and PM2.5mass concentration measurements. These include the Tapered Element Oscillating Microbalance (TEOM(trademark)), and a host of nephelometers such as the DataRAM(trademark) and the DUSTTRACT(trademark). Another nephelometer, the Continuous Ambient Mass Monitor (CAMM(trademark)), only provides measurements of FM. Mass concentration measurements using photometers or nephelometers are based on light scattering, and may be dependent on particle size and chemical composition. Variations in particle size and chemical composition may introduce considerable errors in predicting the response of nephelometers such as the DataRAM.
The TEOM(trademark) measures either PM10or PM2.5(but not directly CM) by recording the decrease in the oscillation frequency of a particle-collecting element due to the increase in its mass associated with the depositing particles. In its standard configuration, the TEOM(trademark) collects particles at a flow rate of 2-4 liter per minute (lpm) on an oscillating filter heated to 50xc2x0 C. The TEOM(trademark) filter is heated to minimize inaccuracies caused by changes in RH that can affect the amount of particle-bound water associated with the collected PM. Determining CM concentrations by difference, as currently proposed by the Environmental Protection Agency (EPA) introduces significant uncertainties in cases where FM account for a large fraction of the PM10. Moreover, since much of the semi-volatile particulate matter (which is mostly associated with FM) may be lost from the TEOM(trademark) filter during and after collection at 50xc2x0 C., there is the potential for a substantially different measurement of PM10mass between the TEOM(trademark) and the Federal Reference Model (FRM). This is most likely to occur in urban areas (or areas affected by urban plumes) where volatile compounds, such as ammonium nitrate and organic compounds can comprise a substantial fraction of the FM. Heating is not likely to affect the mostly non-volatile constituents of coarse particles, thus the accuracy of CM concentrations determined as the difference between PM10and PM2.5may be compromised by the generally random loss of volatile compounds from FM.
In theory, continuous measurements of CM concentrations might be conducted by means of optical, electrical, and time-of-flight monitors. These monitors measure size-resolved particle concentrations based on particle numbers, which could be subsequently converted to volume concentrations assuming spherical particles and an assumption about particle density; both assumptions are required to convert particle volume to mass concentrations. As in most air sampling applications, information on particle density is generally not available and assumptions about its value will introduce uncertainties in the resulting mass concentrations estimates. A far more important limitation of the aforementioned particle number-based monitors results from the sharply decreasing number of ambient particles with increasing particle size. The ambient particle size distribution, by number, is dominated by ultrafine particles (i.e., smaller than 0.1 xcexcm). As well, when converting a number to volume distribution, a 1.0 xcexcm particle weighs as much as 103 times a 0.1 xcexcm particle and 106 times a 0.01 xcexcm particle. Consequently, counting errors associated with this conversion, which may be substantial for large particles, due to their relatively low numbers combined with electronic noise, may lead to significant uncertainties in volume and consequently mass as a function of particle size. This was demonstrated in a recent study by Sioutas et al, which showed that the mass concentrations obtained with the Scanning Mobility Particle Sizer/Aerodynamic Particle Sizer system were higher by 70-200% than those determined with a reference gravimetric method.
A system for monitoring an aeorsol including a plurality of particles is provided. Each of the particles has a size. The system includes an impactor assembly to receive the aerosol at a first flow rate and remove an exhaust portion of the particles that are less than a minimum particle size or greater than a maximum particle size. A remaining portion of the particles is emitted at a second flow rate lower than the first flow rate. A first sensor measures a characteristic of the remaining portion of the particles.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.